Demand for batteries with low costs and high energy density has been gradually increasing in various industrial fields requiring portable electronic devices and energy storage.
A lithium-sulfur battery, which is a secondary battery which uses a sulfur-based compound having a sulfur-sulfur bond (S—S bond) as a positive electrode active material and lithium metal as a negative electrode active material, has attracted great attention as a battery which can replace currently commercialized lithium-ion batteries due to low costs, high theoretical capacity and high energy density thereof. A lithium-sulfur battery stores and produces electrical energy through an oxidation-reduction reaction in which a S—S bond is broken in a positive electrode during a reduction reaction (discharging) and thus the oxidation number of sulfur (S) is decreased, and the oxidation number of sulfur is increased in a positive electrode during an oxidation reaction (charging) and thus a S—S bond is formed again.
However, a lithium-sulfur battery has not yet been widely used commercially because it has a poor lifetime characteristic and low volumetric energy density compared to currently commercially available lithium-ion batteries. Lithium metal, a negative electrode of a lithium-sulfur battery, is known as a material having electrochemically low reversibility and low stability. Meanwhile, a polysulfide, which is a discharging product of sulfur in a positive electrode, moves to a negative electrode, irreversibly reacts with lithium, and thus an active material disappears to decrease capacity. Due to these reasons, it is understood that the lifetime characteristic of a lithium-sulfur battery is poor. When an actual battery is manufactured into a full cell, the volumetric energy density of a lithium-sulfur battery is significantly lower than a theoretical value due to a low content of sulfur in a positive electrode and low current density of a positive electrode.
Before the principle of operation of a lithium-sulfur battery is revealed, it was thought that solid sulfur (S8) particles directly receive electrons to start discharging, and solid lithium sulfide (Li2S) particles generated by discharging the sulfur (S8) directly receive electrons to progress charging. Based on this perception, papers in which sulfur or Li2S is injected into carbon or combined in order to increase electric conductivity by increasing a contact area of a conductive material and a solid sulfur particle (Ji et al., Nature Mater., vol. 8, p. 500, 2009; Zheng et al., Nano Lett., vol. 11, p. 4462, 2011; Yang et al., Nano Left., vol. 10, p. 1486, 2010; Yang et al., J. Am. Chem. Soc., vol. 134, p. 15387, 2012; and Cai et al., Nano Lett., vol. 12, p. 6474, 2012) have been published. During the last five years, research on porous carbon supporting sulfur or Li2S has been highlighted in the lithium-sulfur battery field.
However, currently published papers (Koh et al., J. Electrochem. Soc., vol. 161, p. A2117, 2014; and Koh et al., J. Electrochem. Soc., vol. 161, p. A2133, 2014) have shown that solid sulfur and Li2S particles which are electrically separated from a positive electrode can also participate in the reaction, and thus it was confirmed that electric charges are transferred at a solid-liquid interface not a solid-solid interface. This means that there is no reason to support sulfur in a pore of carbon because the electrochemical reaction of sulfur, which is an active material, is not related to contact with carbon.
Conventional studies have failed to obtain satisfactory effects of improving capacity characteristics of a lithium-sulfur battery and lowering a manufacturing cost thereof due to a high manufacturing cost of porous carbon supporting an active material and a limited amount of an active material that can be supported.